How to animate a bone from it's current position - no matter which one - to a specific one?
If you want to have a better control over the animation you should use a Dope Sheet Editor.
In example below I rotate the arm bone and hand bone. The motion starts from 'idle' position and ends in 'bent' position. The yelow dots in Dope Sheet Editor window represent the keyframes. 


If you want for example to reverse this motion you go to the last keyframe, hover the cursor over the Dope Sheet Editor window, press S and input the value -1. Now the motion starts from 'bent' pose. 


The last thing you may do is to place the animation at the begining of the timeline. You can make it by pressing G button, while hovering the cursor over the Dope Sheet Editor window.